How to get your profile on to a biscuit

Newton and Faraday have left the English banknote, but science will soon regain its lost position when Darwin arrives on the tenner. And in the cheap seats, George Stephenson is still holding out for engineering, appearing, complete with his engine, the Rocket, on the fiver. Having one's picture on a national currency is one of the ultimate status symbols.But anyone who walks around the National Portrait Gallery, home of the exhibition that forms the basis for this beautifully illustrated book, knows that portraiture has many means of conveying and asserting the significance of the people it represents.

In Defining Features , Ludmilla Jordanova explores many ways in which scientists and technologists have been portrayed since 1660, the year in which the foundation of the Royal Society marked the official recognition of the importance of science in national life, though the word "scientist" did not come along for another 200 years. As the Royal Society itself has done for most of its existence, Jordanova includes medical doctors and even some engineers in her survey of the ways in which prestige and influence have been represented.

For us moderns, likenesses of people are easily available through photography, but in the past they had scarcity value, Jordanova reminds us.Even the most famous people were not known by their appearance, as Bonnie Prince Charlie showed by visiting London a few years after Culloden and walking the streets incognito. Portraits of the well-known were not as available as today's television or newspaper images, but they were widely seen by the elite, partly because they were shown off in the houses of the powerful - a semi-public arena in the days when "offices" and residences cohabited in the same buildings.

Jordanova points out that scientists, broadly defined, have been depicted in a wide variety of ways over the centuries. Perhaps the oddest is Dr Oliver, an 18th-century medical practitioner in Bath, whose likeness can be seen in the centre of every Bath Oliver biscuit. Others were remembered via tokens rather than images. Instruments, books and other possessions are obvious collectors' items, but locks of hair, with the resonance of saintly relics, can be found from individuals such as the medical innovator William Hunter. And the importance of the hands to medical practitioners means that these alone have sometimes been represented.

Most of Jordanova's account relates to portraits of a more familiar kind, especially to the changing way in which they represent the power and status of the subject. Many of the devices used - clothes, surroundings, accoutrements, stance - overlap with those used to represent statesmen, soldiers, aristocrats and other leading figures in society. But it would be a mistake to think that the process is all one-way, with settings and contexts lending lustre to scientists that they would otherwise not have.

Instead, the author points out, pictures of figures such as John Hunter (William's brother and also a surgeon) add prestige and the glow of centuries of heritage to the institutions that display them. This applies especially to Hunter's portrait at the Royal College of Surgeons, because it is a splendid work by Sir Joshua Reynolds. So too the bust of William Harvey at the Royal College of Physicians. In this case, Jordanova dwells interestingly on the way in which the institution uses the art it possesses. The college's building in London, designed by Sir Denys Lasdun, has a central well with open-sided galleries filled with artworks. This means that staff and visitors encounter the art from near and far and sometimes at unexpected angles. The college's status is emphasised each time a visitor encounters a bewigged figure from its past.

The style of an institution is also reflected in the way that it chooses to be represented artistically. Jordanova contrasts the University of Cambridge's rather serious, conventional and black-and-white photograph series of distinguished female staff and alumnae, "Educating Eve", with "Portraits of Excellence", a far more varied and adventurous project from the University of Edinburgh, designed to project the university as innovative rather than self-satisfied.

Looking at a series of such portraits, the viewer is struck by the way in which some supply copious scientific material to reinforce their subject while others do without. James Godby's painting of the astronomer William Herschel needs only the night sky as a backdrop. Pictures of Edward Jenner (who has featured on stamps, mugs and bookmarks as well as in oils) often have cows in the background, in honour of their role in the solution of the smallpox mystery.

Herschel and Jenner were heroes of the ages in which they lived, and were depicted as such. Nowadays we have few heroes but many stars, with scientists among them. Jordanova points to Susan Greenfield and Stephen Hawking as scientists with star quality, and might have added geneticist Steve Jones, who like Hawking is well enough known to appear in TV advertising. Stars are almost by definition less formal and more approachable than heroes, as representations of Greenfield prove. Few medals will be struck in her likeness, but thanks to the mass media her influence is exerted nationally and beyond on a scale that few of her predecessors could have achieved.

Martin Ince is deputy editor, The THES . "Defining Features" is at the National Portrait Gallery, London, until September 17 2000.